1. Field of the Invention
This invention relates to a butterfly valve comprising a valve housing with an axial passage for a fluid medium; a valve seat in the form of a seat ring of metal or other material with comparable rigidity, the seat ring being displaceable in the radial direction in a slot in the valve housing and also elastically deformable as to its shape in the radial direction, a throttle arranged so as to be pivoted about an axis of rotation by means of a stem between an open position and a shut-off position, a peripheral seal face on the throttle being pressed against the seat ring in said shut-off position; and means provided for retaining the seat ring in its position in the slot when the throttle is rotated from the shut-off position to the open position and for retaining the shape adapted to the seal face of the throttle which the seat ring has adopted.
2. Description of the Prior Art
A throttle of the above mentioned type is described e.g. in U.S. Pat. No. 4,284,264. This throttle can cooperate with valve seat rings which may have various designs. By way of example the seat ring may have the design which is shown in the said U.S. patent specification or a design disclosed, e.g. in SE-B-No. 445 382. Other designs are conceivable if means are provided to retain the seat ring in the position which the ring has adopted when the throttle has been rotated from the open position to the shut-off position when the valve is being assembled, i.e. at the so called "virgin shut-off movement". It is a common feature of these seat rings that they have a curved sealing surface turned towards the fluid passage. The sealing contact between the two sealing surfaces in the sealing position will therefore occur along a narrow zone and with practically linear contact. In order to obtain simultaneous tightening between the throttle and the seat around the circumference, the known throttle has an ovality in a plane parallel to the side surfaces of the throttle, hereafter referred to as the zero-plane, the major axis being perpendicular to the axis of rotation of the throttle, and thereby also avoiding the throttle touching the seat at an initial phase of the shut-off movement, as well as substantial sliding between the surfaces. For the same purpose the periphery of the throttle has been given a complex double-curved shape, characterized in that the lines of intersection between the throttle periphery and a first plane of intersection through the throttle, coinciding with the axis of rotation and perpendicular to a plane of symmetry through the throttle, consist of arcs of a circle having its centre substantially on the axis of rotation, while the lines of intersection between the throttle periphery and a second plane of intersection defined by the said symmetry plane through the throttle, perpendicular to the axis of rotation, consist of straight lines, the extensions of which will meet, and the curvature of the sealing face of the throttle successively merges from the first mentioned circle in the said first plane of intersection into indefinitely large circles, that is to say, straight lines in the said plane of symmetry. This known butterfly valve has in comparison with previous designs brought about a considerable technical achievement and is today the dominant butterfly valve in Scandinavia, at least in the paper and pulp industry.
However, the above mentioned valve has a few deficiencies. Thus the geometry of the throttle surface is difficult to manufacture with a mathematically correct shape due to the fact that the shape is difficult to program for computerized production. Some approximations must be made in the computer program which means that a mathematically exact shape will not be achieved. In practice this means that the sealing surface on the throttle periphery will have some humps which counteract a simultaneous sealing contact between the seat and the throttle around the whole periphery. Theoretically these problems could be eliminated if one could guarantee that the sealing contact between the throttle and the seat occured exactly in the zero-plane. In practice, however, there are no such guarantees because of manufacturing tolerances, wear from the medium transported through the valve, variations in temperature and torque, etc. Therefore the sealing face on the periphery of the throttle must have a larger breadth than the breadth of the zone of contact in a certain sealing position, so that the plane coinciding with the contact line or contact zone in a certain sealing position may be permitted to form an angle with the zero-plane. Initially this angle is negative, which means that one will achieve the shut-off position slightly before the zero-plane when the throttle is shut off for the first time. Also in this position absolute tightness shall be achieved without subjecting the stem of the throttle to high torques. A good sealing result can be achieved in these positions if one has a high ratio between, on one hand, the surface pressure between the throttle and the seat and, on the other hand, the torque applied to the stem, at the same time as the throttle has a perfect or nearly mathematically perfect elliptic shape in the plane coinciding with the line or zone of contact in each sealing position. These ideal conditions do not exist in the above mentioned, known valve which is characterized in that the lines of intersection between the plane of symmetry and the periphery of the throttle are defined by straight lines. The straight line in the said section will cause a lower surface pressure/torque ratio than what is desirable, and the shape of the line or zone of contact in the said plane which forms an angle with the zero plane will be more or less distorted, resembling the shape of a longitudinal section through an egg.